June has inherited a Caribbean island. As the Challengers fly there, an alien space capsule splashes down. The Challengers rescue an alien creature like an orange raccoon and name it Cosmo. On the island, the volcano is active. The Challengers split up. Prof and Red are almost killed by a rigged palm tree, find the volcano is man-made with oil and smoke, and are trapped by a steel cage. Ace, Rocky, and June run into a gale, except it's caused by a giant fan. Obviously, someone wants them off the island. They get snared in a steel net. Cosmo, the mystery pet, chews through the steel! Cosmo projects eye-beams that disintegrate foliage as they hunt Prof and Red. These two are captured by thugs, and threatened with guns, but Cosmo's paws project force-beams to disarm them. The Challengers brawl. The island was used as a weapons depot by gangs. Cosmo saved the day, so "he's a true Challenger now!"

An eerie light-globe sails into Challenger Mountain, and pops. The guys find themselves in a lab in the mountain - in the 23rd Century! Dr. Oren Gough fetched them accidentally with his time machine. An alarm sounds. The Cosmic Creatures are attacking! Dr. Gough hurls a photon bomb to blind the monsters, and all flee. The Cosmic Creatures came from space seeking selenium, and destroy anything to get it! Selenium is found in meteorites. Red remembers one nearby. They also discover their faces carved in the mountain: a monument from the 20th Century! Dr. Gough explains the entire population of Earth will be evacuated, then a volti-ray will be used to wipe out the CCs. The Challengers volunteer to the ruling council, but Lenroy, a bruising-big scientist, claims the privilege with his three friends. The council rules for eight volunteers, who'll wear protective suits against the volti-ray. But that night, in a restaurant, the Challengers are almost killed by a planted bomb! They rush to see Lenroy, and are stopped cold by superstrong robots! Lenroy plans to save the world and become ruler of the council, then the world. Cosmic Creatures attack. The three robots get trashed. The Challengers fire the volti-rays, incinerating the monsters - except the heroes will die from stray radiation. Nope. Lenroy lied about that, too. He secretly improved the ray to be safe. Dr. Gough sends the Challengers back through time. They agree to not tell anyone about this job, "or they'll think we're off our rockers!"